Many applications of optical fibres required control of either polarisation or the mode shape. Particular examples are found in high-power fibre lasers and amplifiers whose optical output is further processed by polarisation-dependent devices such as modulators and frequency converters. Although polarisation-maintaining fibres exist, these do not necessarily provide a single polarisation output, particularly at high powers where reliable polarising optics can be expensive, unreliable, and difficult to procure. A requirement therefore exists for an active fibre that can be incorporated into a high-power fibre laser or amplifier and which enables the laser or amplifier to be operable as a single-polarisation source.
Stimulated Brillouin scattering provides a limitation for high-power fibre lasers and optical amplifiers. Light travelling down the fibre excites an acoustic wave which reflects the light, the reflected light being shifted in wavelength by the Brillouin wavelength shift. Different glass materials have different Brillouin wavelength shifts and Brillouin bandwidths.
It is known that the stimulated Brillouin scattering threshold can be increased by varying the materials along an optical fibre, by inducing a temperature gradient along a fibre, and by utilizing glasses having different Brillouin shifts across the cross section of a fibre.
It is also known that the stimulated Brillouin scattering threshold can be increased using so-called large mode area fibres. Such fibres can have relatively low numerical apertures and can be operated multi-moded and bent such as to provide higher losses for the higher-order modes as compared to the fundamental mode.
There is a need for a fibre that can be used in single-frequency lasers and amplifiers. There is a related need for a fibre that has a higher stimulated Brillouin scattering threshold.